Atomic Structure & Interatomic Bonding PDF

Engineering Materials Atomic Structure & Interatomic Bonding ISSUES TO ADDRESS... What promotes bonding? What types of bonds are there? What properties are inferred from bonding? Chapter 2 - 1  At...

Engineering Materials Atomic Structure & Interatomic Bonding ISSUES TO ADDRESS... What promotes bonding? What types of bonds are there? What properties are inferred from bonding? Chapter 2 - 1  Atomic Structure Bohr Atomic Model: Electrons are assumed to be revolved (to turn on an axis) around the atomic nucleus in discrete (ayrı) orbitals, and the position of any particular electron is more or less well defined in term of its orbital. Chapter 2 - 2  Atomic Structure atom – electrons – 9.11 x 10-31 kg protons neutrons }1.67 x 10-27 kg atomic number = # of protons in nucleus of atom = # of electrons of neutral species A = atomic mass unit = amu = # of protons + # of neutrons Atomic wt = wt of 6.022 x 1023 molecules or atoms 1 amu/atom = 1 g/mol C 12.011 H 1.008 etc. Chapter 2 - 3  Atomic Structure Electrons have wavelike and particulate properties. – This means that electrons are in orbitals defined by a probability. – Each orbital at discrete energy level is determined by quantum numbers. Quantum # Designation n = principal (energy level-shell) K, L, M, N, O (1, 2, 3, etc.) l = subsidiary (orbitals) s, p, d, f (0, 1, 2, 3,…, n-1) ml = magnetic 1, 3, 5, 7 (-l to +l) ms = spin ½, -½ Chapter 2 - 4  Electron Energy States Electrons... have discrete energy states tend to occupy lowest available energy state. 4d 4p N-shell n = 4 3d 4s Energy 3p M-shell n = 3 3s 2p L-shell n = 2 2s 1s K-shell n = 1 Chapter 2 - 5 Adapted from Callister & Rethwisch.  The Periodic Table inert gases give up 1e- give up 2e- accept 2e- accept 1e- give up 3e- H He Li Be O F Ne Na Mg S Cl Ar K Ca Sc Se Br Kr Rb Sr Y Te I Xe Cs Ba Po At Rn Fr Ra Chapter 2 - 6  SURVEY OF ELEMENTS Most elements: Electron configuration not stable. Element Atomic # Electron configuration Hydrogen 1 1s 1 Helium 2 1s 2 (stable) Lithium 3 1s 2 2s 1 Beryllium 4 1s 2 2s 2 Boron 5 1s 2 2s 2 2p 1 Carbon 6 1s 2 2s 2 2p 2...... Neon 10 1s 2 2s 2 2p 6 (stable) Sodium 11 1s 2 2s 2 2p 6 3s 1 Magnesium 12 1s 2 2s 2 2p 6 3s 2 Aluminum 13 1s 2 2s 2 2p 6 3s 2 3p 1...... Argon 18 1s 2 2s 2 2p 6 3s 2 3p 6 (stable)......... Krypton 36 1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 4s 2 4p 6 (stable) Adapted from Callister & Rethwisch. Why? Valence (outer) shell usually not filled completely. Chapter 2 - 7  Electron Configurations Valence electrons – those in unfilled shells Filled shells more stable Valence electrons are most available for bonding and tend to control the chemical properties example: C (atomic number = 6) 1s2 2s2 2p2 valence electrons Chapter 2 - 8  Electronic Configurations Ex: Fe - atomic # = 26 1s2 2s2 2p6 3s2 3p6 3d 6 4s2 4d 4p N-shell n = 4 valence electrons 3d 4s Energy 3p M-shell n = 3 3s 2p L-shell n = 2 2s 1s K-shell n = 1 Adapted from Callister & Rethwisch Chapter 2 - 9  Atomic Structure Pauli Exclusion Principle: This states that each electron state can hold no more than two electrons, which must have opposite spins. Ground state: When all the electrons occupy the lowest possible energies, an atom is said to be in its ground state. Valance electrons: They occupy the outermost filled shell. Chapter 2 - 10  Atomic Structure Valence electrons determine all of the following properties 1) Chemical 2) Electrical 3) Thermal 4) Optical Chapter 2 - 11  The Periodic Table Columns: Similar Valence Structure inert gases give up 1e- give up 2e- accept 2e- accept 1e- give up 3e- H He Li Be O F Ne Na Mg S Cl Ar K Ca Sc Se Br Kr Rb Sr Y Te I Xe Cs Ba Po At Rn Fr Ra Electropositive elements: Electronegative elements: Readily give up electrons Readily acquire electrons to become + ions. to become - ions. Adapted from Callister & Rethwisch. Chapter 2 - 12  Electronegativity Ranges from 0.7 to 4.0, Large values: tendency to acquire electrons. Smaller electronegativity Larger electronegativity Adapted from Callister & Rethwisch. Chapter 2 - 13  Primary Bonding - IONIC BOND - COVALENT BOND - METALLIC BOND Chapter 2 - 14  Ionic bond – metal + nonmetal donates accepts electrons electrons Dissimilar electronegativities ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4 Mg2+ 1s2 2s2 2p6 O2- 1s2 2s2 2p6 Chapter 2 - 15  Ionic Bonding Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Na (metal) Cl (nonmetal) unstable unstable electron Na (cation) + - Cl (anion) stable Coulombic stable Attraction Chapter 2 - 16  Ionic Bonding Occurs between + and - ions. Requires electron transfer. Large difference in electronegativity required. Example: NaCl Chapter 2 - 17  Ionic Bonding Energy – minimum energy most stable – Energy balance of attractive and repulsive terms − A + B EN = EA + ER = r rn Repulsive energy ER Interatomic separation r Net energy EN Adapted from Fig. 2.8(b), Callister & Rethwisch 8e. Attractive energy EA Chapter 2 - 18  Examples: Ionic Bonding Predominant bonding in Ceramics NaCl MgO CaF 2 CsCl Give up electrons Acquire electrons Adapted from Callister & Rethwisch. Chapter 2 - 19  Covalent Bonding similar electronegativity ∴ share electrons bonds determined by valence – s & p orbitals dominate bonding Example: CH4 shared electrons H C: has 4 valence e-, CH 4 from carbon atom needs 4 more H: has 1 valence e-, H C H needs 1 more shared electrons Electronegativities H from hydrogen are comparable. atoms Adapted from Fig. 2.10, Callister & Rethwisch 8e. Chapter 2 - 20  Primary Bonding Metallic Bonding: It exists in metals and their alloys. The valence electrons in metals are not bound only to their corresponding atom. They belong to the whole metal. The valence electrons in the metal can be regarded as “sea of electrons” or “an electron cloud”. The atomic nuclei form the ion cores. Chapter 2 - 21  SECONDARY BONDING Secondary Bonding (Van der Waals Bonding): It is weak compared to primary bonding. Inert gases has secondary bonding. Secondary bonding arises from atomic or molecular dipole interactions. Hydrogen bonding is a special type of secondary bonding that exist between molecules containing Hydrogen atoms. For example: H2O, HF and HCl, etc. Polar molecules: They have an asymetrical arrangement of positively and negatively charged regions. Example: HCl, HF and H2O. Molecule: It is a group of atoms that are bonded together by strong primary bonds. Chapter 2 - 22  SECONDARY BONDING Arises from interaction between dipoles Fluctuating dipoles asymmetric electron ex: liquid H 2 clouds H2 H2 + - + - H H H H secondary secondary bonding Adapted from Fig. 2.13, Callister & Rethwisch 8e. bonding Permanent dipoles-molecule induced secondary -general case: + - bonding + - Adapted from Fig. 2.15, Callister & Rethwisch 8e. secondary -ex: liquid HCl H Cl bonding H Cl -ex: polymer secondary bonding Chapter 2 - 23  Summary: Bonding Type Bond Energy Comments Ionic Large! Nondirectional (ceramics) Covalent Variable Directional large-Diamond (semiconductors, ceramics small-Bismuth polymer chains) Metallic Variable large-Tungsten Nondirectional (metals) small-Mercury Secondary smallest Directional inter-chain (polymer) inter-molecular Chapter 2 - 24  Properties From Bonding: Tm Bond length, r Melting Temperature, Tm Energy r Bond energy, Eo ro r Energy smaller Tm unstretched length ro larger Tm r Eo = Tm is larger if Eo is larger. “bond energy” Chapter 2 - 25  Properties From Bonding : α Coefficient of thermal expansion, α length, L o coeff. thermal expansion ∆L ∆L = α (T2 -T1) Lo α ~ symmetric at ro α is larger if Eo is smaller. Chapter 2 -  Summary: Primary Bonds Ceramics Large bond energy (Ionic & covalent bonding): large Tm large E small α Metals Variable bond energy (Metallic bonding): moderate Tm moderate E moderate α Polymers Directional Properties (Covalent & Secondary): Secondary bonding dominates small Tm small E large α Chapter 2 - 27  Engineering Materials Atomic Structure & Interatomic Bonding Chapter 2 - 28